This invention relates to an electric power converter which includes switching circuits having power switching elements such as an IGBT and a MOSFET.
As the switching elements have become larger in capacity and higher in speed, power converters using power switching elements have steadily expanded their range of applications. Among such power switching elements, MOS gate type switching elements including IGBTs and MOSFETs in particular have been increasing their field of applications lately.
Non-latching type switching elements such as IGBTs and MOSFETs have a significant advantage of high controllability with gate driving as compared to latching type switching elements such as a thyristor. Non-latching type switching elements can suppress a surge voltage or surge current according to gate control and can control the gradient of the current and voltage arbitrarily even in the period of switching transition at turn-on and turn-off.
One of the applications that make use of the characteristics of non-latching type switching elements is a multiple series high voltage conversion apparatus using an active gate driving technology. The multiple series high voltage conversion apparatus is a high voltage conversion apparatus in which a plurality of elements with stand voltages are connected in series for high voltage applications such as an electric power system. The multiple series conversion apparatus has the problem that slight differences in the switching timing between the plurality of elements connected in series can cause large variations in voltage distribution. A countermeasure against this is the active gate driving technology. This active gate driving technology described above is disclosed in, for example, the following reference: Japanese Patent Application Laid-Open Publication 2005-86940, the entire contents of which is incorporated herein by reference.
FIG. 7 shows an example of a conventional gate driving circuit that uses the active gate driving technology. More specifically, a switching element 9 in connection with a power line 21 has a control input terminal (or a gate terminal). The gate terminal is connected to a voltage amplifier 2 through a gate resistor 3 and the output terminal of a control current source 6. The input terminal of the control current source 6 is connected to the output terminal of a voltage amplifier 5. The collector-to-emitter voltage of the switching element 9, divided by voltage-dividing resistors 4a and 4b, is applied to the input terminal of the voltage amplifier 5.
With such a circuit configuration, in a normal operation state, the switching element 9 makes on/off operations according to the gate signal applied through the voltage amplifier 2. When a surge voltage is generated during the turn-off of the switching element 9, the output current from the control current source 6 increases. The current flowing into the gate terminal of the switching element 9 from the control current source 6 raises the gate voltage of the switching element 9, whereby the collector current of the switching element 9 is increased. As a result, the collector voltage of the switching element 9 drops. The surge voltage of the switching element 9 is suppressed by such an operation.
The foregoing active gate driving technology suppresses the occurrence of a surge voltage by the gate driving circuit making a feedback control on the main voltage Vice of the switching element. Such a system has the advantage of simple circuit configuration since no main circuit element is needed aside from the switching element. However, the system has a problem of increased switching loss, because the switching element bears all the loss. Hereinafter, such a problem will be described in detail with reference to FIG. 8.
When the active gate driving technology is used to suppress a peak surge voltage, extra loss occurs in a period when the surge voltage is suppressed for turn-off. This applies to the period II in FIG. 8. In the period II, the active gate driving circuit operates to clamp the constant collector voltage. The collector voltage during turn-off is proportional to dlc/dt, the time differential of the collector current. In the period II, dlc/dt or the gradient of the collector current has a constant value. Assuming a sufficiently small tail current, the loss E2 of the switching element in the period II is given by:
                              E          ⁢                                          ⁢          2                =                              1            2                    ⁢          Vcep          ×          Icp          ×          t          ⁢                                          ⁢          2                                    [                  Eq          .                                          ⁢          1                ]            where Vcep is the collector-to-emitter voltage in the period II, t2 is the duration of the period II, and Icp is the maximum value of the collector current. In other words, the loss of the switching element in the period II is proportional to the duration of the period II.
Suppose that the active gate driving technology is applied to a power converter that includes a plurality of switching elements connected in series. In such a case, the same collector current flows through all the switching elements connected in series. Since the operation timing varies from one switching element to another, switching elements that turn off earlier bear a higher loss. This will be described with reference to FIG. 9.
In FIG. 9, it is assumed that a single arm constituting a power converter includes three switching elements connected in series. The switching elements have respective collector-to-emitter voltages Vce1, Vce2, and Vce3. The collector current Ic is common to all the elements connected in series. Suppose now that the switching element 1 has a shorter storage time than the other two switching elements 2 and 3, and starts to turn off earlier. In the period I of FIG. 9, the switching element 1 starts to turn off, with a rise in Vce1. The other switching elements 2 and 3 are yet to increase in voltage. Under the active gate control, the voltage Vce1 of the switching element 1 is clamped when a certain voltage Vcep determined by the active gate driving circuit is reached, and the period IIa starts. In the period IIa, the collector-to-emitter voltage Vce1 of the switching element 1 is clamped to Vcep. The other switching elements 2 and 3 start to turn off, whereas the collector current Ic does not significantly decrease yet because the arm voltage is still low. If the number of the switching elements connected in series is large and the voltage for each single switching element to bear is low as compared to the total arm voltage, the collector current Ic drops little in the period IIa. In the period IIb, the voltages of the other switching elements are also clamped to Vcep. The collector current Ic decreases significantly, and then reaches near zero, and the period IIb ends.
As described above, a switching element that starts to turn off earlier consume extra loss during the time t2a as compared to the other switching elements. In the example of FIG. 9, the maximum loss with respect to the switching elements that turn off later is given by:E2a=Vcep×Icp×t2a  [Eq. 2]
The switching speed of a switching element is determined by variations in the switching element's own characteristics, and will not vary if the use conditions such as the gate resistance are constant. Therefore, the switching elements that have higher speeds among the plurality of switching elements connected in series are always the same. Consequently, when the active gate driving technology is applied to a power converter that has an arm including a plurality of switching elements connected in series, switching elements that switch earlier will inevitably consume a higher loss than the other switching elements.